This invention relates to the tread of a pneumatic tire and, more particularly, to the ability of a groove within the tread to eject water.
Tire designers are continually striving to improve tire performance. One goal in improving tire performance is to improve the traction between the tire and the road surface in wet conditions. When a vehicle is travelling on a wet road surface at high speeds, hydroplaning of the tires can occur. Hydroplaning is caused by the tire pushing water in front of it as it advances along a road surface. As the tire continues to push the water in front of it, the back pressure of the water increases and progressively lifts the tire ground-contact area off of the pavement. This back pressure is a function of the depth of the water and the speed of the tire. Eventually, with sufficiently deep water and tire speed, the back pressure lifts the tire off of the road surface. When a tire is hydroplaning, there is no traction between the tire and the road surface and thus, control of that tire is lost.
To prevent hydroplaning, tire designers are continually attempting to improve the ability of a tire to eject or channel water away from the tire. U.S. Pat. No. 5,503,206 discloses a tire having improved wet traction to avoid hydroplaning. The tire that is disclosed in this patent has an annular aqua channel and lateral grooves that direct water from the footprint to either the shoulder area or the aqua channel of the tire where it is ejected away from the tire.
Providing grooves for the water to flow through is the first step in improving a tire""s wet traction. The second step in ensuring that the tire can efficiently eject the water from these grooves. As the tire travels along the road surface, each groove within the tire ground-contact area forms a channel that is enclosed on all sides.
Since each groove within the tire ground-contact area forms a channel, to roughly estimate whether the water flow through each groove is laminar or turbulent, the groove section located in the tire ground-contact area can be analogized to a pipe. The determination of whether flow through a pipe is laminar or turbulent flow is determined by calculating the Reynolds number Re. The Reynolds number Re for flow though a circular pipe is calculated from the equation: Re=xcfx81Dv/xcexc, where xcfx81 is the density of the fluid, D is the diameter of the pipe, xcexc is the dynamic viscosity of the fluid, and v is the velocity of the water. Where the groove and road surface combination does not approximate a circular pipe, the diameter D can be replaced by the hydraulic diameter dh, where dh=4F/U, where F is the cross-sectional area of the opening and U is the perimeter distance around the opening. Generally, if the Reynolds number Re is greater than 2320, then the flow is expected to be turbulent. For example, the flow of water at a temperature of 5xc2x0 C.(40xc2x0 F.) through a 1 cm wide groove on a tire traveling 29 meters per second (approximately 65 mph), estimated using the circular pipe formula, has a Reynolds number Re of 190,789. Thus, the water flow through the groove of a tire travelling at this speed will be turbulent.
Turbulent flow contains eddies or vortices, as shown in FIG. 1 As a result of these eddies, the drag along a surface is higher for turbulent flow than for laminar flow. This drag, known as skin friction drag, decelerates the flow along a surface and forms a boundary layer. Since the flow in the boundary layer is decelerated, the overall flow is reduced.
U.S. Pat. No. 4,706,910 discloses a flow control device that reduces skin friction on aerodynamic and hydrodynamic surfaces. The reduced skin friction is achieved by modifying the micro-geometry of the surfaces by adding riblets or large eddy breakup devices.
U.S. Pat. No. 4,750,693 discloses a device for reducing the frictional drag on a surface of a body in a flowing medium. The surface is provided with an asymmetrical microstructure in the form of a grooved profile.
U.S. Pat. No. 4,865,271 discloses a wall surface with an array of small longitudinal projections or riblets for reducing drag across the surface. The riblets modify the boundary layer flow over the surface to reduce the surface drag.
U.S. Pat. No. 5,133,519 discloses a device that reduces skin friction drag caused by turbulent shear flows of a fluid over a wall surface. The device includes rearward facing microsteps that reduce the drag caused by eddies.
Devices that reduce skin friction drag have received a great deal of attention in recent years, especially on the surfaces of air, water, and land vehicles. The reduction of skin friction drag caused by these devices can result in increased fuel efficiency for aircraft that results in savings of millions of dollars per year. Such devices may also be used in pipelines, as suggested by U.S. Pat. No. 4,907,765. However, drag reduction devices have never been incorporated into tire technology. Although the flow of water through a tread groove may be analogous to the flow of water through a pipe, a tire designer would not look to pipe technology in designing a tread. First, the leading edge of the tire footprint attempts push much of the surface water out of the path of the tire. Secondly, for the water that does enter the grooves, there are three main distinctions between the flow of water through a tread groove and that through a pipe: (1) in a pipe, the water is in motion whereas, in a tread groove, the water is relatively stationary and the groove is in motion, (2) the water flowing through a pipe is in motion relative to all sides of the pipe; whereas, in a tread groove, the water flowing through the groove is in motion relative to only a portion of the enclosed channel since there is little or no motion of the water relative to the ground surface, and (3) in a pipe, the pipe walls remain stationary; whereas, in a tire tread, the surfaces of a groove are subject to vibrations when the tire is in motion. Even when the pressure of the water entering the groove near the leading edge of the tire footprint creates motion of the water forcing it toward the rear of the footprint, the velocity of the water across the road surface if minimal compared to that across the surface of the groove.
International Patent Application Number PCT/JP94/02229 to Fukato disclosed a groove in a tread surface of a tire having a continuously waved bottom surface whose top does not reach the tread surface which claims to increase the ability to discharge water while avoiding an increase in the proportion of the groove. Unfortunately, because such a groove requires a bottom surface, that groove inherently must be very wide to have any effect. The paradox is wide grooves already have the capacity to discharge large volumes of water and resist hydroplaning. Applicants present invention works efficiently on narrow xe2x80x9cVxe2x80x9d shaped grooves having no bottom surface or narrow bottom surfaces. Greatly increasing the value of the invention concept allowing for greatly reduced groove void volumes that are superior in water discharging than conventional grooves.
This invention provides a tire tread for a pneumatic tire. An external surface of the tread has at least one groove for enclosing and channeling water during use of the tire on wet pavement. The groove has at least two surfaces defining a channel. The two surfaces include two side surfaces. The respective side surfaces begin at the external surface of the tread and extend radially inwardly toward an axis of rotation of the tire. The two side surfaces either intersect with one another or with a bottom surface. The groove having a depth defined by an average distance from the external surface of the bead to the intersection of the two side surfaces or to the bottom surface of the groove. A median plane bisects the channel formed by the respective surfaces of the groove. The groove has a width defined by twice an average distance from the median plane to a respective side surface.
The tire tread is characterized by a series of peaks and valleys located on both side surfaces of the groove wherein each valley extends continuously from one side surface of the groove to the other side surface of the groove. An imaginary line or arc located on the media plane within the depth of the groove extends along the length of the groove. At least half of the valleys following imaginary lines skewed with respect to the median plane or arc by an angle or angles in the range of 45 to 90 degrees from the median plane. Each peak has a maximum depth of 15% of the groove width and a minimum depth of at least 5% of the groove width. The depth is defined as the average distance from the peak to the valley.
In a preferred embodiment, the peaks and valleys are on two opposing surfaces of the groove. The peaks and valleys, on the two opposing surfaces, have identical wavelengths and the peaks and valleys on a first surface are 180 degrees out of phase from the peaks and valleys on a second surface.
The inventor believes that the use of the claimed invention can help reduce the undesirable effects of the eddies within the boundary layer of water contacting a groove surface. As a result, the skin friction drag along the respective surface of the groove will be reduced and the flow of water from the groove should be increased.
Definitions
For ease of understanding this disclosure, the following terms are disclosed.
xe2x80x9cBoundary layerxe2x80x9d means the region close to the surface of a solid body over which a fluid flows where the fluid viscosity has an effect. The viscous effect within the region is evidenced by a reduction in velocity of the fluid as the surface is approached.
xe2x80x9cEddyxe2x80x9d or xe2x80x9ceddiesxe2x80x9d means a vortexlike motion of a fluid running contrary to the main current.
xe2x80x9cGroovexe2x80x9d means an elongated void area in a tread that may extend circumferentially or laterally about the tread in a straight, curved, or zigzag manner. Circumferentially and laterally extending grooves sometimes have common portions and may be sub-classified as xe2x80x9cwide,xe2x80x9d xe2x80x9cnarrow,xe2x80x9d or xe2x80x9cslot.xe2x80x9d A xe2x80x9cslotxe2x80x9d is a groove having a width in the range from about 0.2% to 0.8% of the compensated tread width, whereas a xe2x80x9cnarrowxe2x80x9d groove has a width in the range from about 0.8% to 3% of the compensated tread width and a xe2x80x9cwidexe2x80x9d groove has a width greater than 3% thereof
xe2x80x9cLaminar flowxe2x80x9d means streamline flow of an incompressible, viscous Newtonian fluid; all particles of the fluid move in distinct and separate lines.
xe2x80x9cPneumatic tirexe2x80x9d means a laminated mechanical device of generally toroidal shape, usually open torus, having beads and a tread and made of rubber, chemicals, fabric and steel or other materials. When mounted on the wheel of a motor vehicle, the tire through its tread provides traction and contains the fluid that sustains the vehicle load.
xe2x80x9cReynolds numberxe2x80x9d is a dimensionless number that is significant in the design of a model of any system in which the effect of viscosity is important in controlling the velocities or the flow pattern of the fluid.
xe2x80x9cTreadxe2x80x9d means a molded rubber component which, when bonded to a tire casing, includes that portion of the tire that comes into contact with the road when the tire is normally inflated and under normal load.
xe2x80x9cTurbulent flowxe2x80x9d means flow in which the motion of the fluid is subjected to irregular velocities and pressures and results in motion in a random manner. Eddies are located in turbulent flow.